In the field of digital radiography a technique has been developed wherein a radiation image, for example an image of X-rays transmitted by an object, is stored in a screen comprising a photostimulable phosphor such as one of the phosphors described in European patent publication 503 702 published on Sep. 16, 1992 and U.S. Pat. No. 5,340,661.
The technique for reading out the stored radiation image consists of scanning the screen with stimulating radiation, such as laser light of the appropriate wavelength, detecting the light emitted upon stimulation and converting the emitted light into an electric representation for example by means of a photomultiplier.
The signal is then digitized, processed and subsequently used to control the hard copy recording in image reproducing system such as a laser recorder. For diagnostic purposes the reproduction is viewed and analyzed on a lightbox.
Stimulable phosphor screens used in this technique are selected from a variety of sizes which are adapted to the body part that is examined.
The read out apparatus is designed so that it is able to accommodate and read all these sizes of screens.
However, examination types exist in which a larger part of the body is examined than can be recorded on a single, even the largest sized cassette (among commonly used cassette types).
Examples of such examination types are the diagnosis of the entire vertebral column or of the leg, so-called full leg-full spine examinations, where clinical indication requires that e.g. the full spine be examined at once to enable diagnosis or to qualify diagnosis.
In U.S. Pat. No. 5,111,045 and in U.S. Pat. No. 5,130,541 such applications have been dealt with.
U.S. Pat. No. 5,111,045 discloses a method wherein a radiation image of a large object such as a vertebral column is recorded on first and second stimulable phosphor screens that are successively positioned in the image recording unit of the apparatus with their confronting ends superimposed on each other in the recording position. The stimulable phosphor sheets are circulated in the apparatus along a circulatory path through a recording unit, a read out unit and an erasure unit. Special features are provided to hold the sheets in position at the recording unit. After having been exposed, the sheets are consecutively fed to a read out unit where a digital signal representation of the image stored in a sheet is obtained. Then, the image signals representing the images read out of the two sheets are processed so as to combine image portions corresponding to the superimposed portions of the two photostimulable phosphor sheets, with the result that a combined image signal is produced. No details are given on the way in which this image processing is performed.
U.S. Pat. No. 5,130,541 discloses another method and apparatus for recording a radiation image of an elongate body. The method is based on the use of a plurality of elongated stimulable phosphor screens that are successively fed along a circulary path in a combined and dedicated exposure and read out device. The disclosure generally relates to the handling of the sheets in such a dedicated combined exposure and read out device.